How much abuse will the stock locking diff in a 10.5 14-bolt rear from a late model truck take? I've heard that they are not very strong. The one that I have is the Eaton one I belive. It's the deal where it engages when the wheels have a certain percentage difference in speed and it won't engage above around 25 mph...

are you talking about a gov lok? they are pretty cool in principle, but the only thing that is really weak on them is the engagement gear... there is a flywheel with weights on the end, and when the wheelspeed differentiates too much the flywheel spins and when the diameter of the flywheel gets to be a certain distance, it will engage a gear and lock things up.... the gear that the flywheel engages with is known to be fairly weak and you cant buy replacement parts for it... you have to buy a whole new locker :(

I'm sure that the lockers only fit open diffs too....

Yes only fits the open carriers but those are not real expensive. The pinion depth will stay the same and that's about 90% of the pain in the butt of setting up a diff. Get a dial gage and measure the back lash. Put the new carrier in and check it. if it comes out the same your set. If not you will need to buy some shims and play with it a little but setting the back lash is not hard.